The present invention relates to a throttle system for adjusting the suction air flow in an internal combustion engine; and, more particularly, the invention relates to a mechanism that reduces the air leaking through the throttle system while the throttle valve is closed.
Because it is difficult in a throttle system of the type that is used, for example, in an engine, to completely seal the gaps between component parts, air leakage is caused through the gaps between component parts even when the throttle valve is at a closed position. More particularly, in a low speed operating condition, such as in an idling operation, where the air flow required by the engine is less, it is difficult to control the air flow precisely, because the percentage of the total air flow attributed to air leakage through the gaps between component parts increases, while the controllable air flow in the throttle valve decreases relatively. Besides, in order to run an engine at a lower speed, it is necessary to extremely lower the air leakage through the gaps between component parts, because the air leakage determines the minimum air flow of the throttle system.
A known throttle system, such as one disclosed in the Japanese Application Patent Laid-open Publication No. Hei 11-210503, has a construction in which a shaft is supported for rotation in a cylindrical throttle body by anti-friction bearings equipped with a seal mechanism, and a disc-shaped throttle valve is fastened on the shaft so as to rotate with the shaft. This publication indicates that, with this construction, the seal mechanism provided in the bearing operates to prevent air from leaking to the outside of the throttle body.
With a known construction of the type described above, a shaft runs across the suction passage of the throttle body, and the shaft is supported by anti-friction bearings so as to be able to rotate. Because of this, through holes through which the shaft is to be inserted and stepped recessions, that are stepped in the depth direction, for accommodating the anti-friction bearings are formed in the throttle body. The through holes are provided with a larger diameter than the shaft in order to prevent friction, abrasion or seizure, which may be caused in case the shaft contacts the surface which forms the hole. As a result of the above, a gap is formed between the shaft and through hole.
In addition, if the anti-friction bearing is installed in stepped recession of is the throttle body, it is necessary to provide a clearance in order to prevent friction, abrasion or seizure, which may be caused in case the rotating inner ring of the anti-friction bearing contacts the throttle body, and this clearance represents a gap formed by the anti-friction bearing, the shaft and the stepped recession of the throttle body.
The two gaps described above communicate with each other, and, in addition, the former gap communicates with the suction air passage of the throttle body, which represents the primary air passage in which the incoming air into the engine can be controlled. As a result, a throttle system as described above provides a condition such that air can easily flow from the high-pressure upstream side of the suction air passage into the low-pressure downstream side of the suction air passage via a flow passage through these gaps.
Since a lot of air flows from the air cleaner side into the engine suction side through the suction air passage, there arises a problem in that the amount of air leaking into the engine can hardly be reduced in actual practice, even under a condition where the throttle valve is closed and the primary air passage is shut.